WL#3986: Add SHA2 support

We already implement SHA1.  This implements the remaining four fips-approved
Secure Hash Algorithms for computing a condensed digital representation,
SHA-224, SHA-256, SHA-384, and SHA-512, collectively known as "SHA-2".  The
number "224" and such refers to the digest length.

Algorithm specification documents:  
  http://csrc.nist.gov/CryptoToolkit/tkhash.html
  http://csrc.nist.gov/publications/fips/fips180-2/fips180-2withchangenotice.pdf

MySQL SQL function specification:
  SHA2( string cleartext, integer hash_length ) 
    -> string hash, or NULL
where hash_length is one of 224, 256, 384, or 512.  If either is 
NULL or a length is unsupported, then the result is NULL.

  SHA2( string cleartext, integer hash_length ) 
    -> string hash, or NULL

Return a string representation of the hash number in hexadecimal.

hash_length must be one of 224, 256, 384, or 512 (or zero, means => 512).  If
it's none of those numbers, then it must return NULL and raise a warning.

See patch for BUG#13174 "Patch: SHA2 function".